Neutrino di Majorana, the hunt under the Gran Sasso restarts

Scientists of the Gran Sasso National Laboratories toNational Institute of Nuclear Physics (Infn) have taken an important step forward in the search for Majorana neutrino, one of the most elusive and elusive (and important) particles in modern physics. As they tell in an article just published on the pages of the magazine Physical Review Lettersin fact, the experts have successfully completed a “pilot experiment”, called Cupid-0demonstrating that it could be the right key to finally get to identify the Majorana neutrino.

Provided it exists, of course.

Because we are looking for it

Let’s go in order. At the moment, the behavior of all elementary particles we know is described by the so-called Standard Model, a theory formulated half a century ago and successfully subjected to countless experimental tests that have verified its accuracy up to many digits after the decimal point. The Standard Model, while proving correct in predicting the interactions of particles, is nevertheless incomplete, for a number of reasons: first of all it does not take into account the existence of dark matter, which instead (except for surprises) we know to be part of our Universe; moreover, it fails in the description of the so-called asymmetry between matter and antimatter. Basically, we know that each particle has an analogous antiparticle, a copy that is completely identical but with opposite electric charge; we know that matter and antimatter annihilate each other; and according to the Standard Model matter and antimatter should be present equally in our Universe. However, the fact that we ourselves exist, and that all the matter that surrounds us exists, means that at some point there must have been an imbalance between matter and antimatter, in favor of the former. This is where the particle predicted by comes into play Ettore Majorana: “Discover that the Majorana neutrino exists, and find it – he explains Stefano Pirrofirst researcher at the Gran Sasso National Laboratories and spokesperson for the Cupid-0 experiment – it would change the basic theory describing neutrinos, and therefore our understanding of the origin and evolution of the Universe. Demonstrating that the neutrino is a Majorana particle, ie that it coincides with its antiparticle, would justify, in cascade, the abundance of matter with respect to antimatter “. What if the neutrino doesn’t exist? “There are other theories that can explain the asymmetry – continues Pirro – but none are as elegant and compact as Majorana’s. Although it must be said that nature does not necessarily follow elegant and compact theories “.

How we look for it

The search for the Majorana neutrino – or rather, to be more precise, the demonstration that the neutrino is a Majorana particle – passes through the observation of a particular type of radioactive decaythe so-called double β decay without neutrinos (neutrinoless double-β decay, or 0νββ), a reaction in which an atomic nucleus decays without emitting any neutrinos. The problem is that observing a reaction of this type is far from easy, first of all because it is an extremely rare reaction, and secondly because it is a reaction whose effects are very weak, and which therefore can easily be hidden by the “noise “Of other decays. One of the most promising experiments for the search for double β decay is the Heart (Cryogenic Underground Observatory for Rare Events), conducted precisely under the Gran Sasso rock. “Heart – Pirro explains – is an experiment of bolometryin which the temperature increase of a crystal to understand if a double β decay has occurred inside “. The heart of Cuore is composed of 988 crystals of a highly purified natural substance, the tellurium dioxide; the crystals are placed in 19 vertical copper structures, called towers. It is the first solid-state detector of high mass (about one ton), and it is extremely sensitive to the mild energy signal expected for double β decay.

Too much ado about nothing

There is a but: “The problem – continues Pirro – is to reduce the noise from all other reactions, the so-called background noise. In the Heart experiment, the background consists of the so-called α decays, reactions whose energy can ‘cover up’ that of the β decay “. And this is where Cupid-0 comes in. “If you use a crystal that is also a scintillator, a so-called scintillating bolometer, the α decay behaves differently, producing light. In the Cupid-0 experiment we showed that it is possible to identify this light and consequently identify (and discard) all the α decays, thus obtaining a signal without this background noise “. The news, in short, is that Cupid-0 works, and could be the forerunner to a bigger experiment which could allow us, in the future, to finally observe Majorana neutrinos.

From Cupid-0 to Cupid

How much bigger? Quite a lot of. “We have created a international collaboration, Cupid, who plans to set up an experiment a hundred times more powerful than Heart. It is currently in the planning stage: we have written the letter of intent and the technical proposal and we are waiting to raise the necessary funding for the experiment “. In addition to the economic aspect, at the moment researchers are also concerned about the geopolitical situation: “The Cupid-0 crystal was built in Kharkiv – concludes Pirro – and that of Cupid should be built in Russia. If things don’t change it could be a serious problem “.